1. Field of the Invention
The present invention relates to a fluoride crystal, an optical article for an excimer laser using the fluoride crystal, and a method of producing the fluoride crystal.
2. Description of the Related Art
The excimer laser has attracted attention as only one high output laser that can oscillate outside the ultraviolet region, so that the application thereof is expected in the electronics industry, chemical industry, and energy industry.
Specifically, the excimer laser is used in processing metals, resins, glass, ceramics and semiconductors, and in chemical reactions.
An apparatus for generating an excimer laser beam is known as an excimer laser oscillating apparatus. A laser gas filled in a chamber such as Ar, Kr, Xe, F.sub.2, Cl.sub.2 is excited by electron beam radiation or electric discharge. The excited atoms bond with atoms that are in a ground state to produce a molecule existing only in an excited state. The produced molecule is called an "excimer". Due to its instability, the excimer immediately discharges an ultraviolet ray and falls into the ground state. This phenomenon is called "bond free transition". An apparatus for taking out a laser beam by amplifying the ultraviolet ray obtained by the transition in an optical resonator comprising a pair of mirrors is an excimer oscillating apparatus.
Among excimer laser beams, a KrF laser and an ArF laser produce light having a wavelength of 248 nm and light having a wavelength of 193 nm in a vacuum ultraviolet region, respectively. Therefore, a glass material having a high transmittance with respect to light having these wavelengths must be used in an optical system. Fluorites (i.e., calcium fluoride crystals) are preferable as the glass material for the optical system.
The method of producing a fluorite which has been conducted by the present inventors will be explained below. FIGS. 9A to 9D are schematic views showing the steps of the method of producing a fluorite crystal which has been conducted by the present inventors.
In the step of FIG. 9A, a powdered raw material is placed in a container. In the step of FIG. 9B, the material placed in the container is melted and then cooled. In the step of FIG. 9C, solidified agglomerates are pulverized with a pulverizer made of stainless steel. In the step of FIG. 9D, a fluorite block is produced by melting and gradually cooling the pulverized agglomerates placed in a crucible for crystal growth.
The step of FIG. 9B is conducted for reducing the change of a bulk density before and after melting in the step of FIG. 9D, and further for removing impurities in the raw material. For obtaining a higher purity, these steps are repeated a plurality of times.
In the steps of FIGS. 9B and 9D, a scavenger which is a fluoride of a metal is added to the raw material in order to remove CaO generated by the reaction between the raw material (CaF.sub.2) and water or the like, or impurities originally existing in the raw material. For example, a ZnF.sub.2 scavenger reacts with CaO to form CaF.sub.2 and become ZnO and like, and oxygen is removed by removing the scavenger at the time of melting the raw material. As a result, CaO is removed as the impurity to obtain a fluoride crystal having an excellent transmittance characteristic.
The obtained fluorite block is cut in a desired thickness, processed and shaped to be a desired lens shape to be used as an optical article.
Although the conventional fluorite functions satisfactorily as an optical article for an ordinary optical system using a visual light, its optical characteristics deteriorated when radiation of a light beam having a short wavelength and a high output such as an excimer laser was repeated for a long time.
While seeking the cause of the deterioration, the present inventors found out that the cause was derived from not only impurities in the raw material but also the added scavenger. That is, with a large amount of the scavenger to be added for removing impurities such as oxygen, the scavenger itself and a reaction product thereof remain in the crystal of fluorite to deteriorate the internal transmittance and durability of fluorite. However, oxygen cannot be sufficiently removed by merely reducing the amount of the scavenger.